Improving the H_k-Bound on the Price of Stability in Undirected Shapley Network Design Games

TL;DR

This paper improves the upper bound on the price of stability for undirected Shapley network design games with multiple players, providing a tighter analysis and new bounds that advance understanding of game efficiency.

Contribution

It introduces a novel upper bound on the price of stability for undirected games, surpassing previous bounds and analyzing equilibrium conditions via potential functions.

Findings

New upper bound of (1 - Θ(1/k^4)) H_k for k players

Example showing the tightness of previous bounds for 3 players

Improved lower bound on the price of stability to 1.571 for three players

Abstract

In this paper we show that the price of stability of Shapley network design games on undirected graphs with k players is at most (k^3(k+1)/2-k^2) / (1+k^3(k+1)/2-k^2) H_k = (1 - \Theta(1/k^4)) H_k, where H_k denotes the k-th harmonic number. This improves on the known upper bound of H_k, which is also valid for directed graphs but for these, in contrast, is tight. Hence, we give the first non-trivial upper bound on the price of stability for undirected Shapley network design games that is valid for an arbitrary number of players. Our bound is proved by analyzing the price of stability restricted to Nash equilibria that minimize the potential function of the game. We also present a game with k=3 players in which such a restricted price of stability is 1.634. This shows that the analysis of Bil\`o and Bove (Journal of Interconnection Networks, Volume 12, 2011) is tight. In addition, we give an example for three players that improves the lower bound on the (unrestricted) price of stability to 1.571.